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[SVC44-12] Relationship between infrasound-derived and buoyancy-derived eruption cloud volume
Keywords:Volcanic infrasound, Volcanic eruption cloud
The present study examines the relationship between Vinf and Vb for 53 events at 5 volcanoes. We analyze infrasound data accompanying eruptions at Aso, Shinmoedake, and Lokon-Empung volcanoes to estimate Vinf. The data of Vinf examined by previous works at other volcanoes (Sakurajima and Kuchinoerabujima) is also referred. Following the method proposed by Terada and Ida (2007), we estimate F0 with the maximum height of eruption cloud and vertical profile of ambient air density. Vb is then obtained from the value of F0, the gravity acceleration, and density difference between the thermal and ambient air. Referring previous studies focusing on the thermal near the ground, we assume possible density difference as 0.3±0.2 kg/m3.
Our result shows that the ratio of Vb /Vinf is almost always larger than 1 (51 events), and most commonly within a range of 3–3.0×101 (33 events). Since the thermal has the self-similarity, we hypothesize that the relation of Vb /Vinf also follows a linear function. A linear regression function of Vb /Vinf = 1.6×101 is obtained throughout all events. Since examined infrasound waveforms share a prominent pressure pulse at the onset, Vinf likely to ink starting process of the eruption cloud where it is driven by gas thrust, i.e., jets. On the other hand, Vb can be regarded that the volume at the moment when the thermal has entrained enough amount of the surrounding air to ascend with the buoyant force only. Referring the previous works, difference of the bulk density of both eruption cloud regimes yields the volume change rate of the 1.8–3.2×101, that explains our result of Vb /Vinf ratio. Although our result may include errors derived from some factors (the maximum eruption cloud height, the entrainment constant, etc), we believe the result provides an effective index to constrain the eruption cloud volume with infrasound data.